test: fs: Use shared generate_file from utils

test: fs: Use shared generate_file from utils

test_fs/test_erofs.py and test_fs/test_squashfs/sqfs_common.py both
defined a generate_file() helper that writes a file of a given size
filled with 'x'. The two functions were functionally identical and
differed only in parameter names and docstrings.

Move the helper into the existing test/py/utils.py module, which is
the established home for generic test utilities (md5sum_file,
PersistentRandomFile, attempt_to_open_file). Update both call sites
to use it.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: joaomarcos.costa@bootlin.com
Reviewed-by: Simon Glass <sjg@chromium.org>